An emerging obstacle for today's communication systems is scarcity of available spectrum as is disclosed by J. Losey and S. Meinrath, (2010, June). Free the Radio Spectrum, IEEE Spectrum; S. Cherry. (2011, April). The Myth and Reality of Spectrum Scarcity. IEEE Spectrum; R. Hahn and P. Passell, (2011, December). Telecoms and the battle for bandwidth; R. Rubenstein. (2007, February). Radios Get Smart. IEEE Spectrum; and M. Lazarus. (2009), September). Radio's Regulatory Roadblocks, IEEE Spectrum; the contents of which are incorporated herein by reference. Generally, spectrum has been allocated to single users to prevent multiple users from interfering with one another. With the evolution of technology such as the Smartphone and the tablet, modern society increasingly relies on wireless data transmission to function. This has forced many in the wireless market to search for ways to secure more spectrum. AT&T recently attempted to acquire T-Mobile, reporting that its mobile broadband traffic increased 5000 percent over three years. Controversy continues of LightSquared's plans to provide cellular service on spectrum currently reserved for satellite communications as is disclosed by Schneider. (2012, February). LightSquareds GPS-Interference Controversy Comes to a Boil. IEEE Spectrum, the contents of which are incorporated herein by reference. Solutions have been proposed such as license auctions, unlicensed allocation, like Wi-Fi, where devices share spectrum, and operating in white space, spectrum originally left unoccupied by TV channels. The new 802.22 standard attempts to allow multiple users on the same channel by determining which channel can be used without causing interference. A need to find new ways to use spectrum efficiently in wireless communication is increasing.
Various solutions have been proposed to maximize use of spectrum by allowing unlicensed users to share the same spectrum as licensed users. In P. Wang, M. Zhao, L. Xiao, S. Zhous and J. Wang, “Power Allocation Proceedings”, 2007, pp. 4061-4065, the contents of which are incorporated herein by reference, an interactive partitioned water filling algorithm is proposed to fill spaces in spectrum when the primary user is not detected. In G. Zhao, G. Y. Li, and C. Yang, “Proactive Detection of Spectrum Wireless Commun., vol. 8, (9), pp. 4815-4823, September 2009, the contents of which are incorporated herein by reference, a closed-loop power control method is proposed that allows transmission even when the primary user is detected as long as the primary user is outside the interference range of the transmission. In G. Barisal, O. Duval, and F. Gagnon, “Joint Overlay and Underlay Power Allocation Scheme for OFDM-based Cognitive Radio Systems,” in IEEE, 2010, joint overlay and underlay power allocation is proposed.
Given the evolving importance of finding new ways to maximize usable communications capacity, it is worthwhile to explore other methods to share pre-allocated spectrum. The solutions designed to borrow unoccupied space will not be feasible when the primary user is always transmitting. Other solutions require knowledge of position of underlay and overlay transmitters or control over both the transmitters. These solutions do not consider the possibility for multiple underlay transmissions at uncontrolled locations. The concept of the overlay/underlay approach V. Charavarthy, A. Wu, M. Temple, F. Garber and X. Li, “Cognitive Radio Centric Overlay/Underlay Waveform,” in IEEE. 2008 is that the overlay, or overlay signal, occupies unused, white space in spectrum while the underlay occupies underused, gray space in spectrum.
A need therefore exists for a way to select the best frequency for underlay nodes to communicate on to achieve spectral efficiency without knowledge of location of underlay nodes or overlay transmitters and without the availability of white or gray space in spectrum.